I checked TractorHouse, they have more than several complete brake actuators, used and new aftermarket. Used ones don't look too bad, one for $135, another for $161 and change:
368293R91 Parts For Sale | TractorHouse.com
Thanks for the link. I seem to get lost on TractorHouse for some reason.
If the complete assembly is available new for $225, surely some of the subcomponents are available somewhere for cheaper. I may hunt some more, or at least make some noise at some of the vendors. For now I went ahead and welded a washer to the end of the stud and put it back together. Not permanent, but good enough that I can move the tractor and use it for the next few days.
Of course, sometimes it is just fun to play a bit too.
The braking surfaces of my parts are moderately pitted. Since I'm not racing Formula 1 with the tractor, perhaps some pits will be OK. If I get a little better organized, I could try to fill the pits (weld), and machine the surfaces.
These are the parts I snagged from E-Bay. Dirty, and not great photos, but the price seemed good, and they didn't appear to have the rust problems that I have. It will give me some stuff to play with. I'm pretty sure one of the inner seals is leaking, so I need to track that down before I make too big of a mess. Perhaps I can completely rebuild it off tractor then reassemble.
Ebay Brake parts 1
Ebay Brake parts 2